This invention relates to a method for engine speed control via the torque converter lockup clutch combined with other elements of the drive train of a continuously variable transmission.
Hydrodynamic torque converters in automatic transmission perform the task of ensuring a comfortable starting operation with sufficient starting power. Besides, by means of the hydrodynamic torque converter the torsional vibrations of the engine are uncoupled form the drive train. The efficiency losses resulting form the use of a hydrodynamic torque converter are reduced by the torque converter lockup clutch.
In the conventional method for control of the torque converter lockup clutch in stepped and continuously variable automatic transmissions, the torque converter is closed when starting via characteristic lines dependent on rotational speed. Subject to the differences of torque between input and output side, the torque converter is here xe2x80x9cwound upxe2x80x9d at the beginning of the starting operation, which generally results in the generation of a significant rotational speed difference between engine rotational speed and turbine rotational speed. Said difference in rotational speed depends on the torque converter characteristic (soft/hard) and can be distinctly above 1000 revolutions per minute.
In the customary closing of the torque converter lockup clutch controlled by characteristic lines there generates, specially in the lower load range and subject to the generally lower nominal operating point, a clearly negative rotational speed gradient of the engine. The closing of a torque converter lockup clutch is then felt by the driver as an extremely disturbing overshoot, which also adds negatively to the experience.
Furthermore, in a conventional transmission controlled by characteristic lines there generates in the coasting operation, when opening the torque converter lockup clutch, the usual xe2x80x9cdippingxe2x80x9d of the engine rotational speed below the turbine rotational speed, a fact that the driver generally will find disturbing.
The Applicant""s DE 196 31 071 A1 discloses a control system for a continuously adjustable transmission. The engagement or disengagement of the torque converter lockup clutch (WK) is here implemented as function of the control of the nominal rotational speed of the engine. This method makes it possible to reduce the overshoot of the engine rotational speed when starting.
However, the overshoot of the engine rotational speed cannot be entirely dominated thereby: at the start of the regulating phase a distinct increase of the engine rotational speed is to be detected. The cause of said effect is the reaction times of the torque converter to an outer control which amount to a few hundred milliseconds (msec), since the oil rearrangement operation (reversal WK at operation to WK to operation) have to be carried out first.
Only after the oil rearrangement operations have been finished in the torque converter can the torque converter lockup clutch build up the torque needed for control of the engine rotational speed along the trajectory. In an assumed time constant of 40 msec for build up and breakdown of pressure, about 200 msec lapse in this example until the torque converter lockup clutch receives the desired direct connection. The normal average reaction times still are clearly above that. During this time there is no influence on the behavior of the torque converter lockup clutch so that also no control of the engine rotational speed is possible according to this application.
In conventional transmissions of the prior art different circumstances can cause the torque converter lockup clutch not to be fully open whereby a down of the engine is caused when stopping.
Therefore, this invention is based on the problem of providing, departing from the cited prior art, a method for control of the engine speed via the torque converter lockup clutch, which method ensures during the starting phase an immediate reaction of the torque converter lockup clutch and thus optimum comfort with enough starting power, and in the stopping phase prevents xe2x80x9cstallingxe2x80x9d of the engine resulting from a malfunction of the torque converter lockup clutch and the lowering of the engine rotational speed below the turbine rotational speed in coasting operation.
In particular the above mentioned overshoot of the engine rotational speed when starting must be reduced as far as possible.
In addition the inventive method must be of reasonable cost and easy to implement.
It is accordingly proposed to use the closing and opening of the torque converter lockup clutch as an implicit function so as to ensure an optimum comfort by the most asymptotic adjustment possible of the rotational speed gradients of engine and turbine when starting and by a soft, defined separation of the rotational speed curves of engine and turbine when stopping, and thereby to minimize the reaction times of the torque converter by moving the forward clutch while the vehicle stands still to the standstill disengaged mode so that the torque converter lockup clutch can be fully closed.
When starting the engine rotational speed is here approximated by the torque converter lock-up clutch to a defined trajectory, the engine rotational speed being equal to the turbine rotational speed at the end of the trajectory. Consequently, an overshoot of the engine rotational speed when starting is reduced and a jolt-free comfortable closing of the torque converter lock-up clutch is ensured.
According to this invention it is provided that the overshoot of the engine rotational speed at the beginning of the regulating phase when starting be additionally reduced by minimizing the reaction times of the torque converter to an outer control, which can amount to several hundred msec.
Only after the oil rearrangement operations have been finished in the torque converter can the torque converter lock-up clutch build up the torque needed for control of the engine rotational speed along the trajectory. For the earliest adaptation of the engine speed during the starting mode, it will be necessary for the converter lock-up clutch to initiate a direct connection to the system immediately at the beginning of the control phase. The invention suggests to conclude the time-consuming oil transferring processes in the torque converter as early as possible and to keep the torque converter lock-up clutch in active stand-by mode.
This is achieved within the scope of this invention by the fact that the torque converter lock-up clutch is operated when the vehicle stands still in interplay with the forward clutch.
At the same time, the forward clutch must be operated in xe2x80x9cstandstill disengaged modexe2x80x9d (SBC standby control) which, in this range, makes it possible to fully close the torque converter lockup clutch.
In the SBC operation known from the prior art, while the vehicle is stopped, the forward clutch (in position xe2x80x9cRxe2x80x9d applies also to the corresponding shifting element) is opened precisely to the extent that the input torque needed for moving off is in the load-free state precisely somewhat below the outcropping, load torque. As a result is prevented in operation in position xe2x80x9cDxe2x80x9d without actuating the vehicle brake the moving off typical for automatic transmission. By this step in addition to the efficiency losses are reduced due to movement of the operating point of the torque converter. Therefore, the SBC function also brings with it a reduction in consumption.
The power loss and therewith the heat load produced on the torque converter lock-up clutch are hereby outside the critical range.
This procedure eliminates the shifting characteristic lines and the threshold value questions for closing and opening the torque converter lockup clutch.
The defined control of the engine used within the scope of this invention has an advantage in that the compromise between starting power and rotational speed overshoot essential for a positive driving feeling can be implemented in an optimal way.
Besides, the inventive method results in a reduction of fuel consumption.
To ensure when stopping a reliable opening of the torque converter lock-up clutch, there are not admitted, according to the invention, engine rotational speed values below a defined threshold value over the idling speed. It is additionally provided within the scope of this invention to introduce an early opening of the torque converter clutch by means of a boosting of the nominal rotational speed of the engine or increase of the nominal ratio value.
It is further proposed entirely to prevent, by a defined engine rotational speed standard and control of the torque converter lock-up clutch, the xe2x80x9cdippingxe2x80x9d of the engine rotational speed below the turbine rotational speed usual in coasting operations, which the driver generally senses as disturbing. To that end the engine nominal rotational speed is kept as long as possible at the turbine nominal rotational speed. If as a result of disturbing marginal effects (such as resonance humming on account of the almost closed torque converter lock-up clutch in the lower rotational speed range) it is not possible to keep the engine rotational speed value above the turbine rotational speed only by control of the torque converter lock-up clutch, it is also possible here to supportively raise the ratio value.
According to the invention, when the torque converter lock-up clutch cannot be fully opened, the engine stalls when stopping by the fact that the drive train is interrupted upon detection of this malfunction of the torque converter lock-up clutch.
In this case the engine rotational speed trajectory, which is normally used to control the torque converter lock-up clutch, serves as standard value for the slip regulation of the forward clutch.